Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are cube corner sheeting and microsphere-based sheeting. Cube corner retroreflective sheeting, sometimes referred to as “prismatic” sheeting, typically comprises a thin transparent layer having a substantially planar first surface and a second structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits the same total light return regardless of orientation, i.e. when rotated about an axis normal to the surface of the sheeting. Thus, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Various types of microsphere-based retroreflective sheeting are known. For example, U.S. Pat. No. 2,407,680 to Palmquist describes “a reflector structure which by day appears to be continuously coated with paint of one color and by night . . . appears to be continuously coated with a brilliant paint of a different color” (col. 13, lines 57-60). FIG. 1 of the present disclosure is FIG. 6 from U.S. Pat. No. 2,407,680. As shown in FIG. 1 of the present disclosure, prior art microsphere-based retroreflective sheeting 40 includes a flat back reflector 31 having a light-reflective face and a transparent spacing film 32 attached to flat back reflector 31 and bonded to a transparent binder layer 33. A layer of transparent microspheres 34 are at least partially embedded in transparent binder layer 33 such that the back extremity of the microspheres touches or closely approaches spacing film 32 and the front extremity of the microspheres projects beyond binder layer 33. Spacing film 32 and binder layer 33 together constitute a matrix that holds the microspheres in a fixed position and in a spaced relationship to back reflector 31. A pigmented, opaque barrier layer 35 is disposed between the sides of adjacent microspheres. The front surface of the microspheres is visible by diffuse daylight and determines the daylight appearance of the retroreflective sheeting. The nighttime appearance of the retroreflective sheeting is determined by the back reflector.
U.S. Pat. No. 3,758,193 to Tung describes various microsphere-based retroreflective sheetings that reflect infra-red light. FIG. 2A of the present disclosure is FIG. 1 of U.S. Pat. No. 3,758,193. As shown in FIG. 2A of the present disclosure, prior art retroreflective sheeting 10 includes an infra-red transmissive, visible light absorptive layer 11 coated on a microsphere-based sheeting 12. Microsphere-based sheeting 12 includes a support layer 13, a layer of spherical, visually transparent glass beads 14 partially embedded in support layer 13, a reflective layer 15 (e.g., aluminum) covering the back extremity of glass beads 14 to provide a reflective surface that returns radiation through beads 14, and a visibly transparent exterior layer 16.
FIG. 2B of the present disclosure is FIG. 2 of U.S. Pat. No. 3,758,193 to Tung. As shown in FIG. 2B of the present disclosure, prior art retroreflective sheeting 19 includes an infra-red transmissive, visible radiation absorptive layer 20 including a material that transmits infra-red radiation and absorbs visible radiation. In the implementation shown in FIG. 2B, infra-red transmissive, visible radiation absorptive layer 20 includes pigment particles 25. Infra-red transmissive, visible radiation absorptive layer 20 is positioned between visibly transparent glass beads 21 and a reflective layer 22 (e.g., aluminum) covering the back extremity of glass beads 21 to provide a reflective surface that returns radiation through glass beads 21. As shown in FIG. 3, glass beads 21 are at least partially embedded in infra-red transmissive, visible radiation absorptive layer 20. Reflective layer 22 is adjacent to a support layer 23, and glass beads 21 are covered with an exterior layer 24.
U.S. Pat. No. 7,387,393 to Reich et al. (counterpart to Japanese Patent Application Publication No. 2007-171956) describes a low-visibility retroreflective visual tag that includes an infra-red blocking material on a retroreflective substrate. In at least some embodiments, the infra-red blocking material forms a pattern capable of recognition by an infra-red camera when the low-visibility retroreflective visual tag is illuminated by an infra-red light source.
U.S. Pat. No. 4,082,426 to Brown (counterpart to Japanese Patent Application Publication No. S53-68596) describes microsphere-based retroreflective sheeting including markings that are invisible when the sheeting is viewed under retroreflective viewing conditions from a position perpendicular to the sheeting but visible when the sheeting is viewed under retroreflective viewing conditions at an angle. To accomplish this effect, retroreflection in the image areas is reduced. FIG. 3 of the present disclosure is FIG. 1 from U.S. Pat. No. 4,082,426 to Brown. As shown in FIG. 3 of the present disclosure, retroreflective sheeting 60 includes a top layer 61; a layer of transparent, spherical beads 62 (e.g., glass microspheres) supported in a transparent binder layer 63; a spacing layer 64 coated over the back extremities of beads 62 and generally contoured so as to generally follow the curved surface of the back extremities of beads 62; an image layer 65 coated onto spacing layer 64; a specularly reflective layer 66 adjacent to portions of spacing layer 64 and image layer 65; and a back layer 67 (e.g., a layer of adhesive or other polymeric material that protects specularly reflective layer 66, provides a means for attachment of sheeting 60 to a substrate, and/or provides strength to sheeting 60).
European Patent Application No. 0416742 (counterpart to Japanese Patent Application Publication No. H03-75996) describes a license plate including materials selectively absorptive or transmissive in the near infra-red spectrum. Consequently, different images appear when the license plate is viewed in light of different spectral ranges. In at least some embodiments, the infra-red absorptive materials form alphanumeric characters or symbols.
In recent years, the use of microsphere-based retroreflective sheeting has been investigated for a use in license plates. However, in at least some instances, use of microsphere-based retroreflective sheeting results in poor visibility and an inability to read the characters on the license plate due to halation (the spreading of light beyond its desired boundaries in a developed photographic image) when the retroreflective license plate is imaged in an automated license plate reader (“ALPR”) system.
ALPR systems detect and recognize a vehicle using an electronic system. Exemplary uses for ALPR include, for example, automatic tolling, traffic law enforcement, searching for vehicles associated with crimes, and facility access control. One advantage of ALPR systems is that they are can be used almost universally, since almost all areas of the world require that vehicles have license plates with visually identifiable information thereon. However, the task of recognizing visual tags can be complicated. For example, the read accuracy from an ALPR system is largely dependent on the quality of the captured image as assessed by the reader. Existing systems have difficulty distinguishing tags from complex backgrounds and handling variable lighting. One exemplary ALPR system is described in U.S. Pat. No. 7,387,393 to Reich et al. (counterpart to Japanese Patent Application Publication No. 2007-171956). ALPR systems typically use an infra-red camera and an infra-red light source that emits light rays that are incident upon the license plate. The infra-red camera and/or infra-red light source in many ALPR systems is located above or in the vicinity of the road. Consequently, the infra-red light emitted by the camera and/or light source is incident on the license plate at high entrance angles.